Heretofore, as wiring materials for LSI (Large Scaled Integrated Circuits), aluminum or an aluminum alloy has been used predominantly. However, since aluminum has a low melting point (660.degree. C.) and is poor in migration resistance, it is difficult to cope with a higher integration degree and higher operation speed of LSI.
On the contrary, since copper has a melting point of 1083.degree. C. which is higher than the melting point of aluminum and also has low electric resistivity (about ⅔ of aluminum as a bulk value), it is expected as LSI wiring materials for the next generation. However, implementation of copper wiring has several subjects and one of them is an improvement in the reliability of wiring insulative films. It has been known that copper diffuses easily into a dielectric film to lower the reliability (dielectric breakdown voltage) of an insulative film. A method of overcoming the problem has been disclosed in Japanese Patent Laid-open No. Sho 63-76455. In this method, diffusion of copper is prevented by disposing a metal layer as a diffusion barrier on the periphery of copper wiring. However, this method requires steps of forming a metal film as a diffusion barrier layer over the entire upper surface of copper wiring and then removing the metal film of unnecessary portions in order to prevent a short circuit between adjacent wiring. When the distance between the adjacent copper wiring is narrowed, for example, to 0.25 μm or less, it is difficult to remove the unnecessary metal film between the wiring with good accuracy but no consideration has been taken therefor.
The technique as the countermeasure is described in Proceedings of VLSI Multilevel Interconnection Conference, 1993, pp 15. This is a method of covering the upper surface of copper wiring buried in an insulative film with a silicon nitride film as a diffusion barrier layer. Since the silicon nitride film is insulative, there is no requirement for the step of removing the silicon nitride film formed between adjacent copper wiring. In addition, there is no worry of a short circuit between the wiring even when the distance between the copper wiring is narrowed. However, since the specific dielectric constant of the silicon nitride film is as high as about 8 compared with the dielectric constant of general dielectric films (about 4) used in LSI wiring steps, a parasitic capacitance between the adjacent wiring is increased and the propagation speed of electric signals is lowered, but no consideration has been taken therefor.
The technique as the countermeasure is introduced in Proceedings of the 1999 International Interconnect Technology Conference, 1999, pp 109. This is a method of using a BLOk film (BLOk is the trade name of products manufactured by Applied Materials, Inc.) comprising silicon carbide as a main ingredient by using a plasma CVD as a diffusion barrier layer for copper. Since the BLOk film is insulative like the silicon nitride layer, it is not necessary for the step of removing the BLOk film formed between adjacent copper wiring. Further, since the specific dielectric constant of the BLOk film is about 5, an increase in the parasitic capacitance between the adjacent wiring can further be decreased compared with silicon nitride.
Further, another technique as the countermeasure against the increase in the parasitic capacitance between the wiring that is caused when the silicon nitride film is used as the diffusion barrier layer for copper is introduced in Applied Surface Science, Vol. 91 (1995) pp 303-307, and IEEE Electron Device Letters, Vol. 17, No. 12 (1996) pp 549-551. They comprise a method of using an insulative film comprising oxygen, nitrogen and silicon (hereinafter referred to simply as an SiON film) as the diffusion barrier layer for copper. Since the SiON film is also insulative like the silicon nitride layer, it is not necessary for the step of removing the SiON film formed between adjacent copper wiring. Further, since the leak current of the SiON film is relatively approximate to that of the silicon oxide film formed by the usual manufacturing method, there is less worry that the leak current increases between the copper wiring to lower the LSI performance. Further, since the specific dielectric constant of the SiON film is about 5.1 to 5.6, an increase in the parasitic capacitance between the adjacent wiring can further be reduced compared with silicon nitride.
Then, a description is made of another problem that occurs when a silicon nitride film is used as the diffusion barrier layer. In a case where a plurality of layers of copper wiring are integrated on one identical LSI, to establish electrical conduction between different wiring layers, it is necessary to bore a via hole in the insulative film between the wiring layers and bury the hole with a conductor. For this purpose, it is necessary to make an opening in the silicon nitride film as the diffusion barrier layer formed on the upper surface of the copper wiring. However, since the etching rate of the silicon nitride film is lower than that of general insulative films, two major problems arise. A first problem arises in some cases where alignment error is present between the copper wiring and the via hole made thereon. In a case where the via hole has an opening also in a region other than the copper wiring, if the silicon nitride film formed on the copper wiring is etched excessively, since the etching rate of the inter-wire insulative film below the silicon nitride film is high, the interlayer insulative film is engraved. If such phenomenon should occur, burying may sometimes be insufficient upon burying the via hole with the conductor thereby causing conduction failure, or the insulative film of low dielectric constant, when used for the inter-wire insulative film, is denatured to possibly deteriorate the wiring performance. A second problem arises when the thickness of the fabrication mask for the via hole is insufficient upon making the via hole on the copper wiring. When the silicon nitride film on the copper wiring is etched upon making the via hole, the fabrication mask is etched simultaneously. In this case, when the thickness of the fabrication mask is insufficient, the interlayer insulative film below the fabrication mask is etched possibly making the wiring resistance not uniform or the insulative film of low dielectric constant when used below the fabrication mask is denatured to deteriorate the wiring reliability.